Method of screening



June 19, 1956 N. AHLMANN 2,751,979

METHOD OF SCREENING Filed Dec. 30, 1953 2 Sheets-Sheet 1 Zb/{M MK, INVENTOR. Mhh

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METHOD OF SCREENING Nikola Ahlmann, Copenhagen, Denmark, assignor to F. L. Smitlth & Co., New York, N. Y., a corporation of New Jersey Application December 30, 1953, Serial No. 401,266 Claims priority, application Denmark January 2, 1953 2 Claims. (Cl. 209295) This invention relates to the screening of fluent or dry material and is concerned more particularly with a method of screening, in which a gaseous medium under pressure is passed backwardly through the screen for the purpose of freeing the oversize material from the screen openings and thus facilitating the passage of the fines therethrough. The invention is also directed to novel apparatus, by which the method can be advantageously practiced.

Industrial screening operations are frequently carried on by means of vibrating or reciprocating flat screens or by means of rotary screens. In such screening apparatus, the material to be screened is kept in constant motion, so that the fines will pass through the screen meshes and the body of material is moved along the screen surface until all the fines have been removed therefrom. While the apparatus described is satisfactory for screening coarse material, such apparatus When provided with fine screens is subject to the objection that the constant sliding of the residual material on the screen causes heavy wear and also the capacity of the apparatus is low, because the passage of the fines through the small screen meshes is impeded by the residual material. Further, in such apparatus, the material is ordinarily subjected only to gravity and this keeps the capacity of the apparatus low.

For screening fine material, such as flour and pigments, apparatus has been used, which includes a stationary cylinder having its axis either horizontal or slightly inclined and formed in part of screen cloth. Within the cylinder is a central shaft carrying a stirrer which is rapidly rotated. The material to be screened is fed at one end of the cylinder and is advanced therethrough by the action of the stirrer, which throws the material against the screen. In such apparatus, the fines are thrown through the screen meshes under a force greater than gravity and the output is increased but the screen is subjected to heavy wear.

The present invention is directed to the provision of a novel method of screening, which can be carried on more rapidly than the methods practiced in prior equipment described above. In addition, the screen is protected during the practice of the method and the wear thereon is greatly reduced.

In accordance with the method of the invention, the material to be screened is caused to pass through the screen openings by the action of centrifugal force and, at the same time, a gaseous medium is passed through the screen in a direction opposite to the travel of the fines and acts to free the residual material from the screen. As a result, there is less wear on the screen and the screen meshes are kept from being clogged by oversize particles, so that the travel of the fines through the openings is at an increased rate.

In the practice of the new method, the material to be screened may be caused to advance along the screen by the fresh material being fed to the screen and, if desired, the fluid medium, which serves to cleanse the screen by freeing the residual material from the screen openings, may be directed against the screen in such a manner as to promote the travel of the material along it.

2,751,079 Patented June 19, 1956 For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

Fig. 1 is a vertical cross-sectional view of one form of the new screening apparatus;

Fig. 2 is a sectional view on line 22 of Fig. 1;

Fig. 3 is a vertical cross-sectional view of another form of the new screening apparatus;

Fig. 4 is a sectional view on line 44 of Fig. 3;

Figs. 5 and 6 are vertical cross-sectional views of two modifications of the apparatus shown in Figs. 3 and 4; and

Fig. 7 is a vertical cross-sectional view of another form of the new screening apparatus.

The new screening apparatus in the form shown in Figs. 1 and 2 includes a housing 10 having a hopper bottom 11 with an outlet 12, through which the fines produced in the screening operation may be removed. A chamber 14 having a hopper bottom 15 and an outlet 16, is mounted on a side wall of the housing in position to receive the oversize from the screening operation through opening 18.

An inclined screen 19 is mounted within the housing 10 upon spring bearing supports 20 secured to the side Wall of the housing, The lower end of the screen is adjacent the opening 18 leading to chamber 14 and the upper end of the screen is connected by links 21 with an eccentric pin 22 on a disc 23 carried by a shaft which may be driven by any suitable means.

A plurality of conduits 25 extend lengthwise beneath the screen 19 in parallel relation, and a series of nozzles 26 are connected along the lengths of the conduits. The nozzles 26 point generally towards the screen 19, and are inclined towards the lower end thereof. The upper ends of the conduits extend through openings in the housing side wall and are connected to a header 27 which in turn connects with a reservoir for pressure gaseous medium which is ordinarily air. The finely divided material, such as cement raw material, for example, is supplied from a hopper 30 mounted above the housing 10 and connected by chutes 31 and 32 with rotary distributors 33 and 34, respectively, which discharge the material upon the surface of the screen 19.

In the operation of the device described, the jets of air flowing from the nozzles 26 pass through the screen and free the unscreened material from the screen in direct alignment with the jets but, elsewhere, the air passing through the screen does not prevent the fine particles from falling through the screen openings. The nozzles are so formed that the jets assist in moving the residual material toward the lower end of the screen, whence it passes through opening 18 into the chamber 14.

The apparatus of Figs. 3 and 4 includes a housing 35 and a discharge chamber 36 having a common side wall 35a with an opening 37 formed therein. The housing and chamber have hopper bottoms with outlets 38 and 39, through which the separated material and residue may be respectively removed.

A truncated cone structure 40 is supported by a spider 42 upon a horizontal shaft 43 rotatably mounted in bearings 44 and 45. The cone structure is formed of an imperforate material except for a circumferential section which is formed of fine mesh screening material 46. The enlarged open end 40a of the cone extends through opening 37 in partition 35a, and a circumferential flange 47 on the structure adjacent the partition prevents the fines from entering the chamber 36. The opposite end 40b of the structure extends through an opening 48 in the housing side wall and has an opening 49, through which a chute 50 leading from the supply hopper 51, may be inserted. The cone shaft supporting the structure 40 may be rotated by suitable driving means.

A circular header 55 extends about the screen 46 within the housing 35, and pipes 56 extending generally parallel jets is passing in the opposite direction.

to the circumferential surface of the screen and spaced a short distance therefrom, are connected at intervals along the header. Air under pressure is supplied to the header by supply line 57, and flows through orifices formed along the length of the pipes '57 towardsthe screen.

When the structure 40 is rotated, the unscreened material is fed into its interior from hopper 51 through the chute 50. A component of the centrifugal forcegenerated by the rotation of the cone assists in moving the material towards-the discharge opening 37 and the residual material is also urged in this directionby the fresh material entering the structure. The part of the cone structure 40 first coming in contactwiththe material, as it is fed through the chute 50, is a smooth, solid surface, on which the material may be distributed "uniformly before it comes in contact with the screen. The air jets free the material from contact with the screen in those areas in direct alignment with the, jets, but, elsewhere, the fine particles can pass through the screen, even though some air from the As the fine particles pass through the screen 46 they are collected in the hopper bottom of the housing, While the residue passes through the opening '57 into chamber 36.

The modified form of the apparatus shown in Fig.

differs from that shown in Figs. 3 and 4 in that the screen 60 is of cylindrical shape and the screen supporting structure 61 has a tapered end section 61a. Also the nozzle pipes 56 are placed at an angle to the axis of the screen. As the material from the hopper 51 is fed through chute 50 into the interior of the screen supporting structure, it falls upon the interior of the end section 61:: and is dis tributed by centrifugal force evenly about the interior of the end section before it reaches the cylindrical screen 60. Thus, as the screen is rotated in the direction indicated by the arrow, :1 component of force in the direction of the chamber 36 is imparted to the residual material by the air jets. If desired, the nozzle openings may be so formed that the jets point toward the screen surface at an angle and assist in advancing the material.

The modification shown in Fig. '6 includes two truncated cone screen structures 65 and 66 mounted in axial alignment with their ends 65a, 66a of smaller diameter separated by a common partition '67. The opposite open ends 6512, 6650f the conestructures extend through openings 68, 69 in opposite side walls of a housing 70, which communicate with discharge chambers 71-and 72, respectively. The cone structures 65 and 66 are supported by a hub 75 on a horizontal shaft 76 supported by bearings 77 and 73-secured to the outer walls '71aand 72a of the discharge chambers. An annular chamber 79 is mounted against one face of hub 75 and is secured to the common partition 67, and the chamber is divided into a plurality of compartments by radial partitions 80. Alternate compartments communicate with the interior of cone structure 65 through openings 81 in-one wall of chamber 79, while the remaining compartments communicate with the other cone structure 66 through openings 82. Air jet pipes 83a and 83b are equally spaced about the respective screens and disposed parallel thereto, and are connected by conduits 84 with a header 85. The header extends about the smaller ends of the cone structures and is connected with a fluid pressure reservoir.

In the apparatus of Fig. 6, the material from the hopper 51 is fed into the annular chamber 79 through chute S0 and, as the shaft is rotated, the material is evenly fed to the interior of the two cones. The material is moved by centrifugal force towards openings 68 and 69 communicating with the respective discharge chambers 71 and 72, and, at the same time, the fine particles pass through the screens and are collected in the hopper bottom of housing 70. The function of the air jets is the same as previously described.

The form of the new screening apparatus shown in Fig. 7 includes a housing 90 having a hopper bottom 91 with an outlet 92 controlled by a rotary valve 93. The material to be screened is fed from a hopper 94 into the casing 95 of a screw 96 mounted on a shaft 97. The shaft is rotated in bearings, one of which is carried by a standard 98.

A discharge chamber 99 is formed to have a wall 100 in common with the housing 90 and the chamber has a hopper bottom 101 having an outlet 102 with a rotary valve 103. The shaft 97 extends through the chambers and is provided with a second bearing 104 in the outer wall of chamber 99. A circular disc 105 mounted on shaft 97 within housing 90 supports the large end of a screen 1060f truncated conical form, having its small end supported on a bearing encircling casing 95. The disc 105 is provided with openings 105a therethrough and lies opposite an opening in the wall 100. The disc carries a bafiie 107, which extends through the opening in the wall. A disc 108 mounted loosely on shaft 97 is urged to engage disc 105 outwardly from openings 105a by a spring 109 seated at one end against disc 108 and at the other end against the outer wall of chamber 99.

A header 110 supplied with air under pressure from a suitable source, encircles the screen within housing 90 and is provided with a plurality of branches 111 leading to pipes 112, which lie parallel to generatrices of the screen and are provided with a plurality of nozzle openings. The shaft 97 has a passage 113 supplied with air under pressure and leading from one end to outlets within the screen and an air exhaust duct 114 leads from housing 90 and may be connected to the inlet of a dust collector or cyclone separator, indicated generally at 115.

With the construction described, the screen is closed at one end by the material in casing 95 forming a seal and, at the other end, by disc 108 forced against disc 105 by spring 109. The material distributed over the interior of the screen is acted on by centrifugal force and also by air under pressure supplied through the shaft passage 113; the pressure within the screen being greater than the pressure surrounding the screen within the housing 90. The layer of material on the screen is freed therefrom by the action of the jets issuing from the nozzle openings in pipe 112. As the oversize material accumulates in the space between discs 105 and 108, disc 108 is forced backwardly against the action of a spring, and the material escapes from between the discs and falls into the hopper bottom of the discharge chamber. The excess air supplied through pipes 112 and passage 113 is led off through duct 114 and any dust entrained therewith is removed by means of the collector or separator 115.

In the practice of the method in all forms of the apparatus described above, a gaseous medium under pres sure, in the form of jet streams, is passed through the screen openings from the side thereof opposite from that on which the material is distributed. The material is freed from the screen at points in line with the axis of each fluid jet, and the location of the points in the material is constantly changing as the screen is moved relative to the jets. As a result, the layer of material covering the entire surface of the moving screen is subjected to a pulsating action, in which the particles in the layer directly in front of the jets are momentarily dislodged and are returned to the screen upon passing beyond the jets. The dislodgement of the particles from the screen occurs because the jets strike the particles with an impact great enough to overcome the centrifugal force holding the particles on the screen. The particles return at once to the screen after passing beyond the jets because, at the time they are freed from the screen by the jets, the .particles are traveling along circular paths with the screen and such movement causes the particles to be acted on by centrifugal force. Although the particles are dislodged from the screen by jet action, their movement along circular paths continues and they pass beyond the jets and to points, where centrifugal force is acting on the particles and there is no opposing force, such as that of the jets. The unopposed centrifugal force quickly restores the particles to the screen and, as they move back, a rearrangement of the particles over the screen openings occurs and fine particles pass through the openings before the coarse particles block the openings. The dislodgement of the particles from the screen and their rapid return to the screen occur at each roW of jets and constitute the pulsating action of the layer relative to the screen, which enables the fine particles to pass through the screen openings at a rapid rate and makes it possible to screen the material with little wear on the screen, even though the meshes are extremely fine. In order to obtain the pulsating action, it is necessary to coordinate the surface speed of the screen and the pressure of the jets, so that the material supplied to the screen will form a continuous layer covering the entire inner surface of the screen and the jets will be able to overcome the centrifugal force acting on the particles on the screen and free the particles from the screen without moving the particles so far from the screen that they will not return to the screen and form a layer again before the particles pass beneath the next set of jets. In addition to hastening the screening action by creating the pulsations of the layer as described, the jets may be so directed against the screen that a component of the force exerted by the gaseous jets may be utilized to move the residual material across the screen surface towards the point of discharge. The use of the jets in this manner further reduces the wear on the screen, since the material is advanced by the jets toward the discharge while the material is free of the screen.

It goes without saying that the axis of the rotating screen structure may be arranged vertically or obliquely instead of horizontally.

I claim:

1. A method of screening material by means of a rotary screen having the shape of a surface of revolution, which comprises depositing the material on the inner surface of the screen at one end to form a layer covering the entire screen surface, rotating the screen at a speed sufiicient to hold the layer against the screen by centrifugal force, advancing the material in the layer along the screen toward the other end, and causing the layer of material to pulsate relative to the screen by directing air under pressure in a plurality of rows of jets spaced throughout the circumference of the screen against the outer surface of the screen, the air pressure and the speed of rotation of the screen being such that the jets overcome the centrifugal force acting on the layer and momentarily free from the screen the portions of the material struck by the jets, the centrifugal force immediately returning such portions of the material to the screen after the portions of the material have passed the jets.

2. The method of claim 1, in which the jets of air are so directed against the screen that they assist in advancing the material in the layer along the screen.

References Cited in the file of this patent UNITED STATES PATENTS 911,469 Boyd Feb. 2, 1909 2,416,499 Saxe Feb. 25, 1947 FOREIGN PATENTS 23,295 Australia June 29, 1935 

